The present invention relates to an optical wavelength cross-connect for use in wavelength division multiplexed (WDM) systems and, more particularly, to a wavelength cross-connect having reduced complexity.
The next generation wavelength division multiplexed (WDM) transmission systems will carry as many as 80 channels or 400 Gb/s total capacity per fiber. Consequently, large optical cross-connects will be needed in the near future to interconnect multiple fiber transmission lines in a central office. As an example, a cross-connect for 8 incoming fibers and 8 outgoing fibers each carrying 80 channels, will need 640 input/output ports each one capable of accepting at least OC-48 rate and eventually OC-192. Current cross-connects are based on electronic switch fabrics; they first time division demultiplex the high incoming rate (for example OC-48) into lower rates (for example OC-1) and then cross-connect at the lower rate, thus they have a very fine granularity. However, with the emergence of WDM systems that carry large numbers of wavelength channels, a new level of cross-connects with wavelength granularity seem to be highly desirable.
In principle, one can build a much larger cross-connect based on smaller cross-connects. However, to maintain strictly non-blocking characteristics the complexity of a cross-connect in practice generally scales like k2 where k is the number of input/output ports. Thus, a 1000xc3x971000 cross-connect could, in principle, be built out of one hundred 100xc3x97100 cross-connects (the size of the largest currently available cross-connect), but from a cost and size point of view, this would not be practical. Optical cross-connects have been demonstrated by routing the channels in the wavelength domain. An example is the MONET cross-connect [1] (Note in this specification, a reference to another document is designated by a number in brackets to identify its location in a list of references found in the Appendix) However, this type of cross-connect is blocking in the wavelength domain and does not scale well either.
In its most general architectural form, as shown in FIG. 1, a strictly non-blocking optical cross-connect with k input fibers and N wavelength channels per fiber consist of 5 stages [2, 3]; demultiplexing, wavelength interchanging/adaptation, space switching, wavelength inter-changing/adaptation, and finally, a multiplexing stage. The complexity of the cross-connect in FIG. 1 will generally scale with (kN)2 if the fabric is based on space switches only.
Therefore, there is a continuing need to reduce the complexity of the cross-connects used in WDM systems.
The present invention describes a new WDM cross-connect architecture that can selectively cross-connect wavelength channels from any of a plurality of input WDM optical facilities (e.g., fibers) to any of a plurality of output WDM optical facilities. We describe three new cross-connect architectures (apparatuses) that are based on multi-wavelength modules, which are capable of routing simultaneously N wavelengths. In our architectures, the number of required modules scales only with k2 or less (i.e., k2 modules with N complexity) rather than (kN)2 as does prior art architectures. This significant reduction in complexity is traded for a decrease in blocking performance; one of the disclosed architectures (FIG. 7) is strictly non-blocking in the space domain and rearrangeably non-blocking in the wavelength domain, whereas the two others (FIGS. 8 and 9) are rearrangeably non-blocking in both the wavelength and space domain. Another very important advantage of the proposed architectures is their utilization of amplifiers. Since the wavelength channels are optically multiplexed in the interconnection fibers, only a small number of optical amplifiers (one or two per fiber) are needed to compensate for the inevitable transmission loss in the interconnection fabric.
More particularly, our invention is directed to an optical cross-connect apparatus having k input ports and k output ports, k greater than 1, each input port for receiving an M channel wavelength division multiplexed (WDM) signal, M greater than 1, the cross-connect apparatus comprising (1) k wavelength interchange (WI) modules, each WI module connected to couple a WDM signal received at one of the k input ports to a kxc3x97k wavelength selective optical cross-connect (WSC) apparatus, a least one WI module for changing a wavelength assignment of one or more channels of the M channels of the WDM signal received at an input port, and (2) said kxc3x97k WSC apparatus for selectively cross-connecting each wavelength of the k input ports to any of the k output ports of the cross-connect apparatus.
According to other aspects of the invention, the kxc3x97k WSC apparatus may be implemented using (1) k 1xc3x97k splitters and k kxc3x971 combiners interconnected by an array of k2 1xc3x971 WSC elements, (2) a multi-stage Benexc5x9 array of 2xc3x972 WSC elements, and (3) two, one-half Benexc5x9 units interconnected by an array of wavelength interchangers.
In accordance with another aspect of the invention, we describe a method of operating an optical cross-connect apparatus having k input ports and k output ports, k greater than 1, comprising the steps of (1) changing a wavelength assignment of one or more channels of a received M channel wavelength division multiplexed (WDM) signal, M greater than 1, to form a second WDM signal and (2) in response to a control signal, selectively cross-connecting at least two wavelengths of the second WDM signal at at least one of the k input ports to different ports of the k output ports of the cross-connect apparatus.